The performance and life of agricultural machinery parts directly affect the development of agricultural mechanization level in my country. Preparing wear-resistant coating on the surface of agricultural machinery cutting tools is one of the effective methods to reduce wear and reduce economic losses. Laser cladding technology has attracted much attention due to its own advantages. The wear-resistant effects of various wear-resistant coating materials under laser cladding technology were compared, and the problems existing in agricultural machinery cutting tools and laser cladding technology were pointed out. Improvement measures were proposed for the shortcomings of laser cladding technology, and suggestions were made to improve the service life of agricultural machinery cutting tools.

With the development of agriculture in my country, the level of mechanization has been continuously improved, and the development requirements of agricultural parts have become more and more important. Agricultural machinery cutting tools are the top priority of key components of agricultural machinery, which directly affects agricultural development. Mower blades, agricultural deep tillage shovels, plowshares, rotary tiller blades, micro-tillage machine knives, agricultural harvester knives, etc. in agricultural machinery have been in contact with highly abrasive crop trimmings, soil, sand and gravel fragments for a long time, and are affected by wear, corrosion, impact and other damage factors, resulting in the loss of cutting tool materials, causing the blade to become blunt and break. The main reason for the damage of agricultural machinery cutting tools is wear and tear, and the failure rate of agricultural machinery has increased year by year. According to statistics, the average failure interval of domestic combine harvesters does not meet the national requirement of 70 hours, and is only 1/3 to 1/2 of similar foreign products. Every year, 1.5 to 2 million tons of steel are consumed due to wear and tear, accounting for 0.4% to 0.5% of my country’s steel production, equivalent to about 7 billion yuan. It is understood that the damage rate of major agricultural machinery cutting tools has reached about 50%, which seriously affects the performance and life of the machine, and the work efficiency is seriously insufficient. Among them, the service life of the rotary tiller is about 80 hours, and the domestic green shredder blade is about 30 hours. The quality of agricultural machinery cutting tools is difficult to reach the international advanced level. Since the long-term development, the qualified rate of major agricultural machinery parts has continued to be 50% to 60%, and the quality of agricultural machinery parts has become a difficult task. The quality of agricultural machinery parts seriously affects the use of the machine. Therefore, in the technical route formulated by the state in 2015, it is proposed that the manufacturing of core parts is the key to realizing my country’s agricultural machinery equipment. The “Agricultural Machinery Equipment Development Action Plan (2016-2025)” points out that the self-sufficiency rate of agricultural machinery parts will reach 70% in 2025, and agricultural machinery cutting tools are the top priority of modern agricultural development. The development of agricultural machinery cutting tools is a powerful driving force for the progress of agricultural machinery and the primary task of improving agricultural development. At present, both at home and abroad are working hard to research and apply new technologies and processes to improve agricultural machinery cutting tools. Foreign countries expand the development of agricultural machinery on the basis of parts development, advocating the principle of agricultural machinery parts first. Compared with foreign countries, domestic parts research and development capabilities are weak.

At present, the commonly used methods for preparing agricultural machinery cutting tools at home and abroad are surfacing, C N B co-penetration, plasma spraying, laser cladding, flame spraying and electroplating. There are many methods of surfacing, including arc surfacing, plasma arc surfacing, electroslag surfacing, etc. According to the different processes adopted, surfacing is performed to make the surface of the weldment have a special cladding metal layer. Surfacing technology is the most commonly used technology to improve the wear resistance of agricultural machinery cutting tools at this stage, due to its advantages such as good performance, low dilution rate, and small processing allowance. However, it has problems such as large heat-affected zone and easy deformation of workpieces. Therefore, it is picky about the thickness and surface shape of agricultural machinery parts and cannot be widely promoted. The plasma spraying preparation process parameters are complex, the coating contains too many pores, slag inclusions and micro cracks, and the coating is thin; compared with chemical plating and electroplating, it is polluting and cannot be widely used; Zhao Yufeng et al. conducted a study on the wear resistance of the rotary blade surface chromizing process. This process has a certain improvement in improving the wear resistance of the rotary blade, but it also has shortcomings such as easy detachment of the coating, cumbersome process, and environmental pollution. Laser cladding is a surface technology, which refers to the method of pre-coating or synchronous spraying coating on the base material, heating by high-density laser irradiation, so that the coating material and the base material are simultaneously melted to form a molten pool, and then rapidly cooled; molecular diffusion occurs between the coating and the base material, so that the coating layer and the base material surface are metallurgically bonded, thereby achieving the high-performance process requirements of the base material. Laser cladding has the advantages of rapid heating and cooling, metallurgical bonding between the base material and the coating layer, and the coating thickness can reach millimeters and has strong bonding strength. Therefore, this paper reviews the research progress of laser cladding wear-resistant coatings for agricultural machinery cutting tools.

1 Development status of laser cladding wear-resistant coatings for agricultural machinery cutting tools
In the early stage of laser cladding technology, most of the more common iron-based and nickel-based self-melting coating powders were used. Until the late 1970s, some researchers began to explore the use of ceramic powder instead of alloy powder as coating materials. By the late 1980s, the technology of laser cladding ceramic coatings on metal surfaces gradually developed. To date, laser cladding materials can be roughly divided into four categories: self-fluxing alloy powder, ceramic powder, composite powder and other cladding powder.

1.1 Self-fluxing alloy powder
Self-fluxing alloy powder refers to alloy powder with Si, B and other elements added to the alloy as deoxidizers and self-fluxing agents. The advantage of self-fluxing alloy powder is that it has excellent self-fluxing property. The cladding layer and the substrate can be better combined.

1.1.1 Iron-based coating powder
The main advantages of iron-based powder are low price, wide application and good wear resistance. However, iron-based coating alloy has poor self-fluxing property, poor fluidity and easy to have pores. Tian Yongcai et al. [11] studied the wear resistance of iron-based coatings on the surface of rotary blades by laser cladding. They coated iron-based coatings on 65Mn blades. After a series of process adjustments, the microhardness of the cladding layer can reach up to 820H V, which is about 3 times that of the substrate. The wear resistance is significantly better than that of the substrate, the service life of the rotary blade is improved, and the economic benefits are improved.

1.1.2 Nickel-based coating powder
Nickel-based self-fluxing alloy powder is the most widely used, mainly because of its own advantages such as wear resistance and corrosion resistance, and has a high cost performance. However, nickel-based powder has relatively poor high temperature resistance. Ye Pengyun et al. laser clad nickel-based coatings on 65Mn mowing blades. Through orthogonal experiments, it was shown that after the blades were treated, the coating layer formed a metallurgical bond with the substrate, and the microhardness of the cladding layer reached 756HV, which was significantly higher than that of the substrate material, which significantly improved the wear resistance. Yan Yong achieved good metallurgical bonding between the coating layer and the substrate by laser cladding nickel-based coating on the surface of 65Mn agricultural machinery deep tillage shovel, without pores and cracks, and with strong toughness. Its Vickers hardness is 1100HV, which has strong hardness. The wear resistance of its coating layer is three times that of the substrate, which significantly improves the advantages and economic benefits of field operations.

1.1.3 Co-based coating powder
Co-based self-fluxing alloy powder has the advantages of good wear resistance, high strength, good wettability, and still has good corrosion resistance and wear resistance at high temperatures. It is mostly used in high-temperature component materials such as aircraft and ships, but Co-based powder is relatively expensive and difficult to be widely used.

1.2 Ceramic powder
Ceramic powder is divided into pure ceramic coating, metal-based ceramic composite coating and bioceramic coating. It has excellent physical and mechanical properties, such as high strength, high hardness, wear resistance, corrosion resistance, high temperature resistance, etc. Zhang Juanjuan laser clads nano-SiC powder coatings and wear-resistant SiC powder and micron metal mixed powder coatings on 45 steel substrates; after analyzing the cladding layer through friction and wear testing machines, SEM, etc., the hardness of the hardened layer is 900HV, which is higher than that of the substrate material, and the friction performance of the blade is greatly improved, but microcracks will be found in the coating group. Ceramic powder has many advantages, but the thermal expansion coefficient, elastic modulus, thermal conductivity and other properties of ceramic powder are quite different from those of the substrate material. The substrate and coating material are not easy to combine, and defects such as cracks and holes are prone to occur, and the coating layer will be deformed, peeled off and damaged during use.

1.3 Composite coating powder
Composite coating refers to a multiphase solid powder coating synthesized by two or more substances with different physical and chemical properties through certain process means, proportion distribution and mixing methods. Composite coating can ensure the advantages of each component and its relative independence. Composite coating has the advantages of corrosion resistance, wear resistance and high density. Many cutting tools of foreign high-end agricultural machinery and equipment use surface engineering technology, a large part of which uses laser cladding technology. In recent years, the combined harvesters and forage harvesting machinery produced by New Holland and German Claas have improved the wear resistance and corrosion resistance of the cutter base material by laser cladding WC carbide layer coating on the cutter, reducing the loss caused by frequent replacement of cutters and improving economic benefits.

2 Problems with laser cladding agricultural machinery blades

2.1 Causes of failure of agricultural machinery blades
The working environment of agricultural machinery is mostly outdoor operations in a humid or corrosive environment. It often contacts sand and gravel, corrosive liquid on crops and soil, etc., and is severely worn and corroded. It will be damaged to a certain extent during the operation of the machine. Among them, the blade of the rotary tiller in the machine is worn by soil and sand and gravel during operation, and the handle and blade are bent and broken by the impact of soil blocks, stones, etc. The harvester blade is worn and corroded by plant silicates such as straw, sand and gravel, and the thickness of the cutter is relatively thin, which is easy to cause blade chipping. The blade is not stable during cutting, which reduces the cutting rate and operation efficiency. Adhesion wear may also occur between the upper and lower blades. Agricultural machinery blades need to have more effective wear resistance, corrosion resistance, high strength, high hardness and other performance requirements in terms of materials. If these properties are not met, the tool will fail in the early stage. In the complex working environment between fields, the blades are subject to various corrosion and wear, resulting in blade failure and even safety hazards. In addition to sufficient strength, rigidity and toughness, agricultural machinery wear-resistant parts should also have high wear resistance and good corrosion resistance. Agricultural harvesting machinery is specially used to cut plants. Since this part is in contact with highly abrasive crops and lawn mowing, the material gradually loses, which will make the blade blunt and the impact stress will cause it to deform. Under these two conditions, the service performance of the blade will be reduced. In addition, different environmental factors in different places are also an important factor leading to the failure of cutting tools.

2.2 Problems with laser cladding
In the 1980s, some outstanding foreign scholars conducted research on laser cladding. my country began to study laser cladding technology in the early 1990s, which is relatively advanced and has made great contributions to the development of laser cladding technology. It is understood that the strength of components after laser cladding can reach more than 90% of the original strength. Laser cladding is a complex physical, chemical and metallurgical process, but it is a process that is prone to cracks in surface engineering treatment. During the laser cladding process, the rapid heating and cooling of the cladding layer will cause the subsequent solid cooling and shrinkage process to be constrained by the surrounding cooler substrate and produce tensile stress. The magnitude of the stress is related to the hardness of the substrate material. In addition, solid metal will also cause tissue stress due to phase change during the cooling process. Once these stresses are released, they will crack at the weak links. It is more likely to crack in some conventional self-melting materials. Due to the different laser cladding processes, the problems of cracks and pore defects in the cladding process are more uncontrollable. Among them, crack defects are the main problem of this technology. Laser cladding is a process that is sensitive to cracks. Its process parameters include spot diameter, scanning speed, power, powder feeding rate, overlap rate, dilution rate, etc., especially the difference in physical factors such as thermal expansion coefficient and melting point between the coating layer and the substrate is the main cause of cracks. The main root cause of the defect is residual stress, which mainly includes three aspects: thermal stress, tissue stress and constraint stress. The main reason for the generation of thermal stress is that laser cladding is a rapid cooling and heating process. Cooling leads to a large gradient, and there are differences in thermal expansion coefficient and shrinkage rate between the cladding material and the substrate material, which leads to cracks. The formation of tissue stress is due to the phase change caused by the melting of the metal material under heat, which causes the internal stress to exceed the critical value.

3 Laser cladding process solution

3.1 Reduce the cracking phenomenon of the cladding layer by reducing the temperature gradient
Because laser cladding is a rapid cooling and heating process, the coating has a tendency to crack before the cladding operation is completed. This is somewhat different from the general heat treatment process. Therefore, after the cladding is completed, the method of putting it in the annealing furnace has a certain effect, but it cannot affect the residual internal force that has already appeared. Jendrzejewski et al. found that the initial temperature of the substrate will affect the internal stress of the coating. As the substrate temperature increases, the cracks in the cladding layer will be relatively reduced. Li Chunyan et al. adopted a laser preheating method when cladding Ni-based WC coating on the substrate, which significantly reduced the crack tendency of the coating. Studies have shown that preheating can reduce the crack tendency on the one hand, but it will reduce the preparation efficiency.

3.2 Adding a transition layer in the middle
The base material and coating material selected for laser cladding usually have a large gap in physical properties (thermal expansion coefficient, elastic modulus, thermal conductivity, etc.). For example, the thermal expansion coefficient of the nickel-based coating and the thermal expansion coefficient of the carbon steel base, the difference in physical properties between the two will lead to a large thermal stress in the cladding process. If the difference in physical properties between the base and the coating can be reduced, the occurrence of cracks can be effectively reduced. In the study, the occurrence of cracks was reduced by adding a transition layer between the base and the coating. The experiment added a transition coating with physical properties similar to those of the base material, and required the transition layer to have good crack resistance, thereby reducing the probability of cracks and improving the metallurgical bonding strength.

4 Measures to improve the life of agricultural machinery cutting tools

4.1 Cutting tool material selection
Strengthen the research on the wear resistance of agricultural machinery cutting tools. Since agricultural machinery cutting tools often come into contact with soil, sand and gravel, and various plants, the working environment is harsh, the working surface must have high hardness, high wear resistance, and high corrosion resistance, and the cutting edge must have self-sharpening properties. When selecting materials, try to choose medium and high carbon steel, and then heat treat its surface. At present, spring steel and heat-treatable steel are commonly used in China, and quenching and tempering treatment is adopted. Agricultural machinery generally selects heat-treatable steel plates to make carbon manganese boron alloy steel (27MnCrB5) and spring steel (55Cr3). Among them, Thyssenkrupppwo uses XAR steel, which has high strength and high wear resistance and has high potential for development in its agricultural cutting tools. Hoesch Hohenlimburg applies Multibond composite strip steel to harvester cutting tools. The steel is composed of alternating dark layers with high hardness and light layers with good ductility, which can ensure that the cutting tools have high strength and toughness impact.

4.2 Heat treatment strengthening
Heat treatment is one of the important processes in machinery manufacturing. Heat treatment is a heat processing process in which steel is heated to a predetermined temperature in a solid state, maintained at this temperature for a period of time, and then cooled to room temperature at a certain speed. The purpose is to change the internal structure of steel and improve its performance. The mechanical properties of steel can be improved through appropriate heat treatment. For example, the hardness of 65Mn alloy spring steel can reach 400~500HV after quenching and medium-temperature tempering. The appropriate heat treatment process can eliminate the defects caused by hot processing processes such as casting and forging, refine the grains, eliminate segregation, reduce internal stress, and make the structure and performance of steel more uniform.

4.3 Wear-resistant coating material strengthening
On the basis of laser cladding, by selecting different coating materials, the wear and corrosion failure of cutting tools can be effectively reduced. Different wear-resistant materials have different properties. Select a wear-resistant coating suitable for the requirements of the cutting tool, in which the coating is required to have the advantages of high hardness, high wear resistance, high hardness at high temperature, and sufficient strength and toughness. The Department of Metal Technology Forming Technology of Paderborn University proposed the use of wear-resistant materials, functional coatings and local application strengthening mechanisms to coat a relatively hard layer of wear-resistant materials on a relatively soft cutting edge. Since the cutting edge is affected by wear conditions, the wear-resistant layer has better performance.

5 Conclusion
(1) Agricultural machinery is an important support for the development of modern agriculture. Agricultural machinery parts are an important part of agricultural mechanization, and the development of agricultural machinery cutting tools is even more important. With the continuous development of agriculture, the life and performance of agricultural machinery wear-resistant cutting tools are increasingly unable to meet the pace of modern agricultural development, especially in some high-end agricultural machinery equipment.
(2) To improve agricultural production efficiency, it is necessary to improve the reliability of agricultural machinery equipment, promote the development of agricultural machinery parts, and strengthen the research on wear-resistant, corrosion-resistant, high-strength and high-toughness materials. At the same time, strengthen the strengthening of materials. Only by improving the innovation and upgrading of key parts of agricultural machinery can the international competitiveness of my country’s agricultural machinery equipment be improved.